In recent years, non-woven fabrics comprising synthetic fibers, by making the best use of characteristics of those fibers, are extending their applications into various fields, such as clothing materials, medical materials, engineering and building materials, and materials for industrial use.
Among them, non-woven fabrics containing PTFE fibers are excellent in heat resistance, chemical resistance and abrasion resistance, and are expected to be further developed as highly functional non-woven fabrics.
Cotton-like PTFE materials being made into the non-woven fabrics are gathered PTFE fibers, and so far have been made in such manners as mentioned below:
(1) A process for producing filaments and then cutting to a desired length. PA1 (1a) An emulsion spinning method disclosed in U.S. Pat. No. 2,772,444. PA1 (1b) A method disclosed in JP-B-22915/1961 or JP-B-8769/1973. This method comprises stretching of fibers obtained by slitting a PTFE film to a desired width. A problem of this method is that when the slit width is made narrower, the finer the obtained fiber is, the more easily the fibers are broken at the time of stretching. PA1 (2) A process for preparing PTFE fibrous powder in the form of a pulp and making a sheet-like material therefrom by paper making process (U.S. Pat. No. 3,003,912 and JP-B-15906/1969).
The process for producing PTFE filaments is roughly classified into the following two processes.
This method comprises extrusion spinning of a viscose binder, and the like containing PTFE particles, and then sintering to obtain the filaments having a uniform figure, the section of which is defined by a shape of nozzles. Major problems of that method are such that a binder remains as a carbonaceous residual after sintering, the obtained PTFE filaments are colored in a dark brown, and even if the carbonaceous residual is oxidized to be discolored, an original purity cannot be maintained. The method has also a drawback that since a complicated step is employed, cost is high.
Both PTFE fibers obtained by the methods (1a) and (1b) have a low friction coefficient and a high specific gravity inherent to the PTFE, and therefore are not intermingled sufficiently with each other even if having been crimped. (JP-B-22621/1975)
The method of the above-mentioned U.S. patent is to cut PTFE rod, cord or filament obtained by a paste extrusion, to a short length and to apply a shearing force to obtain PTFE fibers.
JP-B-15906/1969 discloses a method for making fibers by applying a shearing force to the PTFE powder.
Any of the fibrous powder obtained by the above-mentioned methods can be made up to a sheet-like material by paper making process but cannot be made into a non-woven fabric by the use of a carding machine, needle punching machine, or the like as they are short in fiber length and in the form of a pulp.
In order to eliminate those drawbacks of the PTFE fibers, the present inventors have developed a method of producing PTFE staple fibers (relatively short fibers) and cotton-like PTFE materials directly by tearing to open a uniaxially stretched PTFE film with mechanical force and filed the patent application (JP-A-78264/1993). The PTFE staple fibers obtained by that method are bulky and contain fibers having good intermingling property but also contain a lot of short fibers which do not contribute to enhancement of intermingling property. Those short fibers drop in a carding step for producing non-woven fabrics, and thus yield is not good.
An object of the present invention is to provide a method of producing cotton-like PTFE materials containing PTFE fibers having excellent intermingling property.
Another object of the present invention is to provide PTFE split yarns having a network structure, which are usable for producing the cotton-like PTFE materials, and a method of producing the PTFE split yarns.
Yet another object of the present invention is to provide a PTFE filament usable for producing the cotton-like PTFE materials and a method of producing the PTFE filaments.
Yet another object of the present invention is to provide a filter cloth for dust collection which can be obtained from the cotton-like materials produced by the above-mentioned method of producing the cotton-like PTFE materials.